1. Field of the Invention
This invention relates to a logical circuit which is constructed of insulated gate field-effect transistors (hereinbelow termed "MISFETs").
2. Description of the Prior Art;
Logical circuits which are constructed of MISFETs (hereinafter termed "MIS logical circuits") are broadly classified into a ratio type logical circuit and a ratioless type logical circuit. In the former, the output level is determined by the ratio g.sub.mr between the mutual conductances of a load MISFET and a driving MISFET, so that the gate area of the driving MISFET becomes large. Also, a d.c. current flows through the load MISFET and the driving MISFET, so that the power dissipation becomes high.
In the latter, the output level is determined independently of the ratio of the mutual conductances as stated above. More specifically, a circuit of this type precharges a load capacitor and acquires an output level in dependence on whether or not the stored charge is discharged in conformity with an input signal at the next timing. Therefore, the gate area of a driving MISFET is small, and the power dissipation is low. On the other hand, however, a clock pulse circuit for the precharging is required.
In the logical circuits as described above, a supply voltage is sometimes applied between the drain and source of the driving MISFET constituting a logical block which is connected between an output terminal and a reference voltage terminal (earth terminal). For this reason, the channel length of the driving MISFET must be determined in dependence on the supply voltage in order to prevent punch-through between the drain and source.
Accordingly, the ratio type logical circuit must have the ratio g.sub.mr between the conductances of the load MISFET and the driving MISFET at a predetermined value and must be designed in consideration of the limitation of the channel length of the driving MISFET. The ratioless type logical circuit cannot put the driving MISFET into a required minimum size due to the limitation of the channel length, and must be designed in consideration of the supply voltage.